1. Field of the Invention
The present invention relates to a cell voltage balancer with the aim of equalizing voltages of a plurality of cells configuring a battery pack.
2. Description of the Related Art
A plurality of cells (here, a cell means a chargeable secondary battery) are serially connected to configure a battery pack. Even when such a battery pack is charged, the cells therein may vary in voltage. This is caused by a difference in degree of progress of charge among the cells even when the battery pack is charged at a constant current due to the presence of variations in value of internal resistance and the like among the cells. A device for eliminating the variations in cell voltage as thus described is generally called a “cell voltage balancer”.
A circuit is loaded on the cell voltage balancer. A configuration of the circuit is under studies from a variety of aspects. For example, there is a cell voltage balancer where a pair of a resistor and a switch that are serially connected with each other is parallelly connected with each cell (Japanese Patent Application Laid-Open No. H10-050352). The pairs of the resistor and the switch are, for example, controlled in the following manner so as to equalize voltages of the cells.
(1) When a voltage of a cell reaches a charge target voltage, a switch concerning the cell is switched to a closed state, to bypass a charging current through the pair of the resistor and the switch. Thereby, the cell is discharged so that voltages of the cells are equalized.
(2) When variations in voltage among the cells are large, a switch concerning a cell with a high voltage is switched to the closed state, thus to bypass the charging current through the pair of the resistor and the switch. Thereby, the cell is discharged so that the voltages of the cells are equalized.
(3) A microprocessor constantly monitors a charging state of each cell from information on measurements of a voltage, a current and the like of each cell to finely control opening/closing of the switch, so as to prevent large variations in voltage of the cells. Thereby, the voltages of the cells are highly accurately equalized.